In steam turbines, solid particle erosion damage to both stationary and rotating components in the steam path is a significant problem. The problem is exacerbated as the average in-service age of a steam turbine increases. A principal source of such erosion damage is the existence of iron oxide particles in the steam resultant from the exfoliation of oxides formed on the inner surfaces of the steam boiler tubes and steam piping at elevated temperatures and which particles impact on the nozzles and buckets along the steam path. Solid particle erosion damage in steam turbines in general is a major contributor to problems associated with operation and maintenance of steam turbines, for example those used by utilities to generate electrical power. These problems include loss of sustained efficiency, forced outages, extended maintenance outages, cost of maintenance, cost of replacement parts and shortened inspection intervals.
Solid particles are prevalent during start up of the steam turbine particularly at bucket tip locations in the early stages of the high and intermediate pressure sections of the turbine. Significant erosion damage can occur as these particles work their way down the turbine. Damage typically occurs to the bucket cover, the seals for which are typically labyrinth seals, as well as to the diaphragm, nozzle and bucket airfoils. When brush seals are used as bucket tip seals either alone or in combination with labyrinth type seals, solid particle erosion upstream of the seal can become more severe as solid particles are not able to pass through the brush seal as easily as they pass through a labyrinth seal. Additionally, the solid particles become entrapped in the interstices of the bristles and/or between the bristle pack and the upstream backing plate. This can lead to further damage to the brush seal and other parts of the turbine.
A variety of different designs have been proposed and utilized to reduce solid particle erosion in the steam path of a turbine, particularly in areas of the turbine other than seals. For example various coatings have been applied to the buckets and nozzle partitions. However, bucket tip labyrinth seal designs generally have not been altered to reduce solid particle erosion as it has been assumed that particles will eventually flow through the labyrinth seals. However, because brush seals typically have tips which lie close to or engage the rotating component, solid particles may be precluded from readily passing through the brush seal and will become trapped in upstream areas of the bucket or shroud or in the bristles per se leading to damage to the turbine. Accordingly, there is a need to design a brush seal which will alleviate the solid particle erosion problem in connection with brush seals, particularly those used in steam turbines.